In a copending application Ser. No. 138,400 of Robert Henry McCarthy, filed Apr. 8, 1980, for DYNAMIC IMAGE ENHANCEMENT AND APPARATUS THEREFOR and assigned to the assignee of the present invention, there is disclosed a digital subtraction radiographic system for image enhancement of low contrast objects utilizing venous injection of a contrast material to visualize a difficult-to-image anatomic region of interest. In the digital subtraction system, disclosed in the copending application, a radiographic digitized composite mask image of a region of interest of a patient is acquired and stored in a first digital memory. During the clinical procedure, a quantity of contrast material, preferably an X-ray opaque dye, is introduced into the patient. Depending on several factors including the type and quantity of contrast material, physical characteristics of the patient, and the distance between the remote vein and the region of interest, such as a carotid artery, the time for the contrast material to take effect varies, typically between approximately 10 and 15 seconds. After the contrast material takes effect, a second radiographic image substantially of the same region of interest as the reference image is acquired and stored in a digitized form in a second digital memory. Alternative techniques are disclosed for subtractively combining the two digitized images to cancel the overlapping background, thereby enhancing the region of interest which shows the contrast material. The resultant subtracted image, known as a difference or differential image, devoid of a significant amount of non-useful background information, permits superior clinical evaluation of the difficult-to-image region.
A clinical investigator about to perform a diagnostic study involving the acquisition of a series of radiographic images generally will not know beforehand the length of time for the contrast material to take effect in the anatomic region of interest after injection since there are so many factors in this determination. Thus, an image acquired early in the study, typically the first such image, will be designated the reference or mask image. Each subsequently acquired image then becomes a candidate for the post contrast image to be subtracted from the mask to yield the desired difference image.
The ideal difference image is generated from the pair that permits the greatest amount of cancellation of background or non-useful information and yields the most information in the region of interest. The first condition requires a high degree of overlap of the two images which is determined principally by the degree of movement in the imaged region. The second condition is a function of the concentration of contrast material in the region of interest and this is a function of time. Patient movement, both voluntary and involuntary, has posed a sufficient clinical problem that the investigator would prefer to use as the mask, an image which comes as close in time as possible prior to the appearance of a substantial quantity of contrast material in the region of interest to significantly minimize motion between the time of the acquisition of the mask and the acquisition of the post contrast image. Hence, the resultant difference would provide the desired high contrast in the region of interest with maximum overlap and hence cancellation of the non-useful background.
For any given mask, the system disclosed in the aforementioned copending application, permits generating a series of difference images by subtracting from the mask, independently and sequentially, any other image, i.e. a post contrast image, and viewing the difference. Though a trained physician can typically and rather expeditiously select the best difference image once a mask is preselected, the result will not necessarily yield the best difference image available in the study since the original choice of mask may not have been the best since the choice was at best an informed guess. Moreover, even if this process were generally acceptable when performed by an experienced investigator, the technique is not likely to consistently yield acceptable clinical results when operated on a grand scale.
The procedure of introducing contrast material into the object studied is complex and must be coordinated with the operation of an X-ray system and data collection system. To require concurrent analysis and decision making on the quality of images acquiring burdens the operators and the system. The present invention permits the data to be collected over a long enough time interval to insure the collection of the best mask and data (contrast) image. Then, as a leisurely post-processing operation, one can select the best pair of images to subtractively combine. Furthermore, since the contrast material does not necessarily reach all parts of the image at one time, it is doubtful whether the a priori selection of a single mask would be adequate to an analysis of the entire image.